Ultrahigh frequency transmission line termination



Nov. 11, 1941. G. H. BROWN ULTRAHIGH-FREQUENCY TRANSMISSION LINE TERMINATION Filed Aug. 3l, 1938 mw H Bnventor calyawmzow/ dttorneg sistive acteristic over a Patented Nov. l1, 19.41.

'or-Fics i ULTBAIIIGII FREQUENCY TRANSMISSION LINE TERMINATION George H. Brown, Haddoniield, N. J., assignor to Radio Corporation of America, a corporation of `Dela Appllcltlon August 31, 1938, Serial No. 227,864

4- Claims.

My invention relates to terminations for transmission lines and more specifically to terminating loads for ultra high frequency power sources.

An alternating current transmission line has a characteristic impedance. lI1' the resistance of the line per unit length is negligible, and sifch is the usual case, the characteristic impedance where L=the inductance per unit length and C=the capacitance per unit length of the line. If the line is terminated in a resistance equal to the characteristic impedance, reflections from the terminating impedance will be avoided. In lines used for currents of a few cycles per second up to several million cycles per second, it is not a difficult problem to terminate the line in a resistance. When the frequencies extend from about thirty megacycles per second upward, as a practical matter, a pure resistance (i. e. one free from reactance is practically non-existent for power dissipation exceeding a few watts).

One of the objects of the invention is the provision of means for terminating ultra high frequency transmission lines.

Another object is minating an ultra line such thatv the tc provide means ior terhigh frequency transmission line termination will be a reload capable of substantial power dissipation.

An additional sion line with a termination consisting of a second transmission line having input impedance equal to the characteristicimpedance of the first line, vand having a substantially resistive charrange of frequencies.

The invention will be described by reference to the accompanying drawing in which Figure i. is a circuit diagram oi'ra resistlvely terminated transmission line;

Figure 2 is a circuit diagram of a transmission line terminated in a prcpertiesz` Figure 3 is an' illustration of one embodiment cf the invention; and

. Figures t and 5 are graphs used in speciiicalhy applying the invention.

In. Fig. l. a pair ci conductors I, having an input connected to a supply of alternating current, are terminated in a resistive load 3. If the lines have an inductance L and a capacitance C, their characteristic impedance object is to provide a transmis- I resistor having reactive If the load is chosen so that its resistance in ohms equals the characteristic impedance in ohms, undesired reections from the terminating load will be avoided.

In Fig. 2 the transmission line 5 is terminated by a resistor 1 which has'reactlve properties represented by thedash line 9, Il indicating respectively inductance and capacitance. -If high frequency currents are applied the inductive` reactance or the capacitive reactance will predominate at all but the resonant frequency. Any attempt to tunetout the predominating reactance, will simply narrow the frequency range at which the termination is resistive. Furthermore, most practical applications require a resistive load over a band of frequencies and in many installations it may be necessary to dissipate considerable power.

Although the foregoing discussion has been applied to open conductor transmission lines, it holds equally true for concentric lines. In a concentric line of the type shown in the left portion I3 of Fig. 3 the characteristic impedance currents of a frequency of 30 mc. and for the vdissipation of several kilowatts may be solved in 'accordance with my invention by terminating the first or conventional line in a second line having an input impedance equal to the characteristic impedance of the first line and having a similar frequency characteristic. If the sec-1 ond or terminating line has a negligible resistance per unit length, it would have to be iniinitely long to have the required attenuation and propagation characteristic. To avoid this practical difficulty, a line having a conductor or conductors having the required resistance per unit length is used.

The characteristics of a transmission line having a highly resistive inner conductor are de- 1termined as follows:

At a reference frequency fn the transmission line resistance per unit length is R=resistance per unit length, kh=constant, en -.Zum c=ve1ocity of light, Ao=wave length, L=inductance per unit length, C=capacitance I per unit length, and Zo=charaterlsti imped- MCB \ istic is: 1

For the short cli-cuites une it has been snown Comm cation Engineering," edition of By successively setting k=25, 15, and 5 (or any arbitrary values) the phase angle 4 may be determined for a number of ratios of hlx. These values are applied to the graph of Fig. 4, which gives the phase'angle for any selection of length h to wave length A for the choseny constant k. In a like manner graphs, Fig. 5, may b e plotted for the ratio Zmput/Zc to the ratio h/x for the several values of constant k. The application of these `graphs will hereinafter appear.

Again referring to Fig. 3, the nrstconcentric line I3 is connected to a second or terminating line I8. The second line has an inner conductor I9 which has' the required resistance per unit length. The inner conductor is preferably made by coating the outer surface of a ceramic or other suitable hollow insulated form with graphite, carbon or like conductor. The terminating ends 2l may be silver or copper plated to make low resistance connections to the ground 23 andtermination of the first line. The diameters of the inner and outer conductors of the second line are preferably equal respectively to those of the nrst line. The hollow form of the conductor I9 permits connections for cooling water, which may be used for power dissipation.

The selection of similar conductor diameters means that Zmput/Zo will equal unity. (See Fig. 5.) Therefore: for k==,25, h/),=0.0066; for k=15, h/ \=0.01 and for 7c=5, h/)\=0.032.

Now referring to Fig. 4 if the immediately foregoing h/A values are applied to the several curves respectively, it may be seen that the phase angle is positive and fairly low and constant over a range for ltr-25, h/)\=.0066. The same is true for k=15, h/).=.01. For k=5, h/).=.032, the phase angle is 7.5+. It follows that k=l5 and h/7\=.01 is an appropriate value.

By way of example, the termination for a line operating at 45 mc. and having a characteristic impedance of '10 ohms (a==11/4"; `b=4V2") consisted of the values indicated above. Since the total resistance should equal 'I0 ohms and since t X=m Cm.

and h/ .=.01. h=6.67 cms. Therefore the termination of a standard 'I0 ohm line operated at 45 mc. may be a cencentric line o'f conductors of the same diameters and a length of 6.77 cms. with a total resistance uniformly distributed in the length h of 'I0 ohms. By water 'cooling the resistor, several kilowatts of power may be dissipated.

, angle of the second line is negligible.

' more, the resistor isarranged for water cooling 1932, by W; L. Everett, page 113. Equation 15,;

yzo

Thus the invention has been describedy as a termination for an ultra high frequency transmission line. The termination. is a second line which includes a conductor having high resistance per unit length. The resistance is chosen so that the total resistance vequals the impedance of the first line, and so that its phase Furtherin order that the power dissipation may be high. It will be observed that the grounding ofA the resistor makes theY line especially suitable for water cooling. Instead of using the second line for terminating `a standard line, it may serve other purposes. For example, the second line may be used as a load for a dummy antenna. Of

course, it should be understood Sthat the invenillign applies equally well to open or concentric I claim as my invention:

1. A pair of ultra high frequency transmission lines, the first of said lines having a pair of conductors having negligible resistance per unit length and a characteristic impedance zog@ where L=inductance and C=capacitance, and a second line connected to said first line, said second line including a conductor having high resistance per unit length whereby the input impedance of the second line equals the characteristie impedance of the nrst line and the characteristic impedance is equal to where Ic=a constant, w=2rffand w0=21rfo.

2. A pair of ultra high frequency transmission lines of the character described in claim l including means for dissipating a substantial amount of power. J

3. Apair of ultra high frequency concentric transmission lines, the first of said lines having a pair of concentric conductors having negligible resistance per unit length and a characteristic impedance L z0 x/g where L=inductance and C=capacitance, and a second line connected to said first line, said second line including a conductor having high resistance per unit length whereby the input impedanceof the second line equals the characteristic impedance of the first line and the characteristic .impedance is equal to .kwa l ZC Zg 1 '-1 w where k=a constant, w=21rf and wc=21rfc- 4. A pair of ultra high frequency transmission lines of the character described in claim 3 including means for dissipating a substantial amount of power in said conductor of high re- GEORGE H., BROWN. 

